The present invention is directed to a process for a production of a coated glass body, in particular, for the production of glass fiber light waveguides or conductors. The process comprises forming glass layers on a surface of a rotating body in a cycle which includes applying a layer of a substance which either contains glass or is transformed into glass and subsequently transforming each applied layer of the substance into a glass layer by locally heating and moving the zone of local heating along the surface of the body in a given direction.
An example of a process of a type in which a glass forming substance or glass substance is deposited on a surface of a body and then transformed into a glass layer is a chemical vapor deposition (CVD) process. An example of such process is described in an article by P. Geittner et al, "Low-Loss Optical Fibers Prepared by Plasma-Activated Chemical Vapor Deposition (CVD)", Applied Physics Letter, Vol. 28, No. 11, June 1, 1976, pp. 645-6, and also in the articles mentioned in the footnotes 1-3 of this article. In a process of this type, the glass body consists of a glass tube whose inner wall surface is coated by conducting a flow of gas through the rotating tube and strongly heating a short portion of the tubes by means of a heating device so that a chemical reaction can take place which leads to a depositing of a glass forming substance out of the gas. The heating device is slowly moved in a longitudinal direction of the tube so that the heating zone in the tube slowly travels along the length of the tube.
A glass fiber formed from the internally coated tube of this type or kind is produced in a known manner in which the tube is deformed or collapsed into a rod. The rod is then heated at one end to the necessary temperature so that a fiber may be drawn from the heated end of the rod. The fiber formed by this process has a stepped index profile.
However, the fibers produced by this method have been proved to have the properties of being either depolarizing, birefringent, or optically active.
Practical applications for fibers frequently required fibers which are neither depolarizing, birefringent nor optically active. For example, monomode glass fibers, which can be produced by a process as described above, would be predominantly suitable for the transmission of short light pulses in communication technology if the propagation delays between the two orthogonally polarized states of the fundamental mode in the fiber did not occur and thus undesireable reductions in the information transmission were not present in the fiber. Many other applications of the fiber will require a predetermined polarization of the emerging light. This occurs in interferometric processes such as a fiber optical rotation measurement process and in magneto-optical processes such as a fiber optical current strength measurement process.
The origin of the undesired properties of the real glass fibers includes deviations of the fiber core from the cylindrical symmetry or the anisotropic property of the material. A process for increasing the cylindrical symmetry is disclosed in German O.S. 26 25 010 and consists that during the deformation of the glass tube such as collapsing or drawing the tube into a rod, an increased gas pressure is provided in the remaining part of the tube and is maintained. A fiber, which has been improved in this manner in respect to a cylindrical symmetry, does in fact exhibit a clear reduction in birefringence in comparison to a fiber of lesser symmetry, but a residual birefringence and an optical active property still remains. From this, the inventors of the present invention have deduced that the anisotropy still exists in the coating materials.